The present invention relates to monopulse radar and more particularly to the monopulse radar traditionally used for airborne weather detection.
Anticipated increases in air traffic volume, as well as economic pressure to reduce airline operating costs are spurring the development of an air transportation system that operates at maximum capacity under all visibility conditions. Several hazards to the safety of flight, however, present themselves or are exacerbated during reduced visibility operations. These hazards include: impact with the terrain surrounding the airport; failure to acquire the intended runway for landing; and failure to detect obstructions that may be on the runway, taxiway or otherwise in the path of the aircraft. For these reasons, the air traffic control system imposes minimum cloud ceiling and runway visibility requirements at airports that mandate under what conditions the airport can accept landing and departing traffic. These minima are designed to ensure that the flight crew has enough information to acquire the correct runway and avoid collision hazards on departure and approach.
In addition, low visibility operations at airports also require that air traffic control separate landing and departing traffic from each other by a greater distance. The net effect of the increased separations is to reduce the number of aircraft the airport can handle in a given time period.
Relaxation of the given minima at an airport is possible if both the aircraft and the airport have sophisticated precision guidance equipment. The precision guidance equipment (e.g. instrument landing system (ILS) or microwave landing systems) improves the confidence with which the aircraft can acquire and maintain the proper flight path to the correct runway. Airports having this precision guidance equipment can enjoy improved capacity during times of low visibility over airports without this equipment. However, this equipment is expensive to acquire and maintain and many airports do not have equipment of this type. Furthermore, these systems require specialized equipment both on board the aircraft and at the airport. In addition, use of these systems still do not provide the airport with the same capacity present during times of unrestricted visibility as hazards to flight due to the reduced visibilities still exist.
Certain dedicated systems are currently manufactured to warn of these potential hazards. Chief among these systems are those designed to prevent controlled flight into terrain accidents. Controlled flight into terrain accidents currently account for the greatest number of air fatalities, the risk of which is greatly increased by operations in low visibility conditions. Technology for avoiding controlled flight into terrain includes ground proximity warning systems, and terrain awareness and display systems.
Ground proximity warning systems use altitude information from radio altimeters and barometric altimeters, in conjunction with an individual aircraft""s speed and climb characteristics, to warn flight crews that the terrain below the aircraft is rising dangerously fast. The ground proximity warning systems can also provide an aircraft flight crew with additional alerts by, for example, warning of aircraft deviation below glideslope or inappropriate aircraft attitude or configuration. Typical examples of ground proximity warning systems are disclosed in U.S. Pat. No. 3,946,358 entitled xe2x80x9cAircraft Ground Proximity Warning Instrumentxe2x80x9d and U.S. Pat. No. 4,914,436 entitled xe2x80x9cGround Proximity Approach Warning System Without Landing Flap Input,xe2x80x9d both incorporated herein by reference.
Terrain awareness and display systems combine ground proximity warning system technology with navigation data, a built-in terrain data base and existing cockpit display technology such as color weather radar, electronic flight instrument systems (EFIS) and map displays. Terrain awareness and display systems provide xe2x80x9clook aheadxe2x80x9d terrain warnings by utilizing present aircraft positions and a terrain data base to predict the aircraft""s future position with respect to terrain. A typical example of a terrain awareness system is described in co-pending application Ser. No. 08/509,642, filed Jul. 31, 1995, entitled xe2x80x9cTerrain Awareness Systemxe2x80x9d by Muller et al, and assigned to the same assignee as the present application.
Although the ground proximity warning systems and terrain awareness and display systems described in the above-mentioned references have greatly reduced the controlled flight into terrain risk for aviation worldwide, both ground proximity warning systems and terrain awareness and display systems have some limitations. Neither of these systems actually xe2x80x9cseesxe2x80x9d the terrain or other obstructions ahead of the aircraft. Ground proximity warning systems differentiate the aircraft""s altitude signals to detect abnormally high closure rates with terrain. Thus, discontinuities in the terrain profiles, such as a cliff, may not generate an alert in sufficient time to prevent an accident. The more sophisticated xe2x80x9clook aheadxe2x80x9d function of terrain awareness and display systems compares aircraft position data, based on either dead reckoning or a global positioning system, with a stored terrain map to calculate the aircraft""s probable position relative to the terrain and determine whether a terrain collision threat exists. However, this system cannot detect collision threats due to obstructions not contained within the database. For example, temporary structures such as construction cranes would not be modeled in the database. In addition, the integrity of the alerting function depends directly upon the integrity of the aircraft position data. Errors in aircraft position could reduce the warning time given the flight crew. In addition, non-fixed terrain features and non-fixed terrain threats such as, for example, aircraft or vehicular traffic on the runway, are also not readily determinable by typical ground proximity warning systems. Thus, these systems are inappropriate as a means for relaxing airport visibility minima and increasing airport capacities.
Radar has the potential to provide the flight crew with real-time terrain information independent of both a calculated position and a computer-stored terrain data base. However, the only radar normally carried aboard non-military aircraft is weather radar. Weather radar has characteristics that make it non optimal for detecting terrain threats specifically. Existing weather radar antennas exhibit a limited elevation sweep angle. The added weight and expense of a radar dedicated to terrain detection in addition to the already required weather radar prohibits use of terrain only radar systems. Yet, additional safety and increases in airport capacity could be realized through the use of this additional radar information.
Utilizing an aircraft radar for detection of these threats, poses unique difficulties. Effective real-time terrain feature identification and terrain threat determination require resolution of closely spaced targets, for example, closely spaced radio towers. The typical monopulse radar antenna transmits through the Sum channel and receives data through Sum and Delta channels. Nearly all current radar applications utilize only these two Sum and Delta channels, which are manipulated to obtain target off-boresight angle information. Radar applications utilizing the Sum and Delta channels exclusively are incapable of resolving closely spaced targets. If multiple targets are present in the beam or if the target is widely distributed, the monopulse measurement becomes confused. Traditional monopulse angle measurement is unable to separate two closely spaced targets; known monopulse sharpening techniques may even degrade the image. Thus, it has been difficult to distinguish multiple targets or widely distributed targets when they are concurrently present in the radar beam. Resolving closely spaced targets is valuable in achieving radar-based autonomous landing guidance.
The present invention responds to the need for clear weather operational capabilities in low visibility conditions by providing navigation, guidance, and terrain/obstacle collision avoidance information. The navigational and threat avoidance capabilities of the present invention also serve to enhance the safety of flight regardless of the visibility conditions. More particularly, the present invention provides a system that provides landing guidance to an aircraft through use of data collected using the aircraft""s onboard radar. Data collected and processed according the invention includes: terrain and obstacle data, runway and airport verification, navigation and position data and runway touchdown and threshold information. In this manner, the present invention can provide the flight crew with an accurate representation of the airport and of any collision threats actually present ahead of the aircraft even in conditions where the crew cannot obtain this information visually for themselves. Thus, the invention enables aircraft landings in ceiling and visibility conditions which would otherwise be below the airport minimums and can be employed to greatly increase airport capacities.
In a preferred embodiment of the invention, the invention utilizes the aircraft""s weather radar, thereby avoiding the cost and weight penalties of an additional radar system that heretofore has prevented the use of such guidance systems on civilian aircraft. The present invention is implemented in such a manner as not to compromise the weather and windshear detection functions the weather radar is also required to perform.
According to one aspect of the invention, the invention uses the weather radar, typically in the X-band, to detect terrain and obstacles ahead of the aircraft. Thus, the present invention provides information to the flight crew about the actual obstructions present, rather than the virtual obstructions represented in a terrain data base or inferred through differentiation of radio altitude signals. The present invention can therefore alert the crew to hazardous conditions such as runway vehicular traffic and temporary structures not normally contained in a terrain data base.
In one embodiment of the invention, the present invention utilizes this detection of targets to provide ground proximity warnings to the crew. In another embodiment of the invention, the present invention utilizes the detection of targets to alert the crew to runway incursions on the intended runway for landing or during taxiing operations. The ground proximity warning function of the present invention, can function independently of, or be used in conjunction with, other ground proximity warning devices to enhance the robustness of the terrain alerting system.
According to another aspect of the invention, the features detected by the radar are used for positive identification of the runway and/or airport to which the aircraft is on approach. Features detected by the radar can be compared with a stored data base of features for the given runway and airport to confirm that the aircraft is approaching the desired airport and the desired runway on that airport.
According to still another aspect of the invention, the present invention further employs the airport and runway correlation feature described in the preceding paragraph to detect the aircraft""s navigation system errors. Augmenting the aircraft navigation data with the radar information facilitates greater precision in the approach and landing phase of flight. In a preferred embodiment of the invention, this updated position information can optionally be displayed to the crew.
According to yet another aspect of the invention, the invention utilizes monopulse beam sharpening of the radar signals to further enhance the ability of the weather radar to be used as a terrain and obstacle detection radar. The monopulse beam sharpening function of the present invention utilizes a new distributed monopulse channel, Delta_D, thereby improving the resolution of multiple targets in the radar beam. The invention can distinguish multiple or close spaced targets by time multiplexing traditional monopulse channel with the Delta_D channel to provide more accurate angle measurement for targets in each side of the beam. In a preferred embodiment of the invention, the invention utilizes novel monopulse beam sharpening techniques for even greater resolution of targets within the beam. Thus, the present invention overcomes the limitations of using weather radar for terrain, obstacle, and feature extraction.
According to yet another aspect of the invention, the navigation, guidance and/or collision information acquired and/or updated via the weather radar, is displayed on an aircraft cockpit display, such as a head-up display. Other visual displays may also be used. For example, the display may be an overlay of the cockpit weather display or an EFIS display. The display provides a synthetic image of the runway that conforms to the actual runway as viewed from the cockpit. The synthetic runway image can also contain other symbols such as, for example, a flight path indicator, an extended runway centerline, and touch down zone symbols. These symbols may optionally be fused with data generated from other sensors. The display operates to improve the situational awareness of the flight crew to enhance safety and to facilitate landing in low visibility conditions.
The various features of the present invention can be used to reduce the visibility minima at airports without requiring the airport itself to acquire expensive or specialized equipment. The reduced minima can in turn, increase the capacity of the airport during periods of reduced visibility. Thus for example, aircraft equipped with the present invention can land at airports having only Category I visibility minima (Decision Height (DH) 200 feet and Runway Visual Range (RVR) 2,400 feet) when the conditions actually present are Category III (Decision Height (DH) 0 feet and Runway Visual Range (RVR) 700 feet) and the airport would normally be closed.
Similarly, the present invention supplements the Traffic Alert and Collision Avoidance System by providing warnings of potential runway or mid-air collisions, even when the intruding aircraft is not equipped with an Air Traffic Control Radar Beacon System transponder.
According to still another aspect of the present invention, the invention may be incorporated and used in conjunction with other aircraft functions such as existing ground proximity warning devices or to facilitate emergency descents.
Other features and advantages of the present invention are described in greater detail below.